Guide to Basic Electronics Theory--Meters

In practical electronics work, it’s very often necessary to measure the basic parameters in a circuit (that is, voltage, current, and resistance) You can calculate the nominal values, but component tolerances can throw the actual values off quite a bit. For example, a circuit containing three resistors, each with a tolerance of ±20% can be off value by as much as ± 60% (actually, it’s very rare for the values to be that far off, because some of the component errors will probably cancel each other out; but it’s possible.) Also, the calculations for complex circuits can be extremely tedious, time- consuming, and difficult to keep track of.

Various defects can cause components to change their values drastically, so for troubleshooting a method of actually measuring the circuit values is absolutely essential. It’s the only way to pinpoint the defective component.

Fortunately, there is a relatively simple way to measure voltage, current, and resistance. These measurements are made with devices called meters.

The D’Arsonval meter

By far, the most common type of meter is the D’Arsonval meter. The construction of this kind of meter is shown in FIG. 1.

A permanent magnet in the shape of a horseshoe is positioned around a coil of wire wound on a piece of soft iron. This coil—which is called the armature, or the movement of the meter—is on a pivot that allows it to move freely. When an electrical current flows through the coil with the polarity shown, the movement develops a magnetic field. Its north pole will be facing the north pole of the permanent magnet, and its south pole will be facing the south pole of the permanent magnet. Because like poles repel, the movement will turn on its pivot so its poles will no longer face the poles of the permanent magnet. Just how far the movement will turn will depend on the relative strength of the magnetic fields involved.

--- -1 Construction of a basic D’Arsonval meter. Spring; Permanent magnet; Armature; Coil electromagnet.

The magnetic field of the permanent magnet is constant, of course, but the magnetic field of the armature will depend on the strength of the electrical current flowing through it. Therefore, the motion of the armature will be directly proportional to the amplitude of the applied electrical current.

A pointer can be attached to the center of the armature, so that the amount of applied current is indicated on a calibrated scale. This scale is marked in equal units spaced so that the current value can be read directly.

A small spring opposes the rotation of the armature. This small amount of physical opposition is easily overcome by the applied current and its resulting magnetic field, but when current stops flowing through the coil, the spring forces the movement (and thus, the pointer) to return to the zero position.

The current applied to this type of meter must have the correct polarity If the polarity of the current is reversed, the south pole of the electromagnet (armature) will be facing the north pole of the permanent magnet, and vice versa. Because unlike poles attract, the armature won’t move.

Some meters can be adjusted so that the zero position is in the center of the scale. This zero position will allow the pointer to move in either direction, thus indicating current of either polarity Most meters, however, don’t have this capability

A meter movement is quite delicate and fragile. Meters are usually enclosed in protective cases (made of transparent plastic), but they can still be permanently damaged if dropped or otherwise mishandled. Also, the coil in the armature is made of very fine wire so it will be light and move easily, This means the wire cannot carry much current. If too much current is applied directly, the armature can be quickly ruined. Fortunately, there are methods of decreasing the current actually applied to the meter, and still allowing the meter to give an accurate reading. This subject is discussed in this section. With a reasonable amount of care, a D’Arsonval meter movement is sturdy enough for practical use and can last for years.

This type of meter (which is sometimes also called a moving-coil movement meter) is quite popular for a number of reasons. It’s relatively inexpensive, highly accurate and sensitive (that is, you can readily measure very small currents). The scale is uniform and easy to read, because the movement of the pointer is directly proportional to the applied current, resulting in evenly spaced calibration markings. A D’Arsonval meter drains very little current from the circuit being tested, so it’s quite efficient, and doesn’t significantly affect the values being measured. Finally, the D’Arsonval meter can easily be adapted to read current, voltage, or resistance. This type of meter is so commonly used, that whenever a meter is mentioned in electronics work, it can generally be assumed to be a D’Arsonval type unit unless otherwise specified.

Ammeters

When a meter is used to measure electric current, it’s referred to as an ammeter (from ampere meter). The standard schematic symbol for a meter is shown in FIG. 2A. Letters are generally placed within the circle to indicate exactly what kind of meter is being used. For example, the letter A in the symbol in FIG. 2 B means the unit is an ammeter.

- --2 Schematic symbols. Any meter (A); ammeter (B); milliammeter (C); microammeter (D).

Because the ampere is a rather large unit for an electronic circuit, milli-ammeters ( FIG. 2C), or micro-ammeters ( FIG. 2D) are frequently used instead. All three types of ammeters work in exactly the same way—the only difference is the range of values they are capable of measuring.

To measure current, the meter must actually be inserted into the circuit itself. In other words, the meter is placed in series with the circuit. This arrangement is shown in FIG. 3. Remember that in a series circuit the current is equal at all points, so if an ammeter is placed in series with the rest of the circuit, the same current that flows through the circuit will flow through the meter.

- --3 An ammeter in series with the circuit to be measured.

Because the meter is in series with the circuit, its internal resistance must be as low as possible, to avoid upsetting the normal current flow. For example, suppose you have a circuit with a total resistance of 10,000 11 and is powered by a 10 V source. The nominal current will be equal to E/R 10/10,000 = 0.001 A (1 mA).

When the meter is inserted in the circuit, its internal resistance will add to the resistance of the circuit. For instance, if the meter internal resistance is 5000 ohm the total resistance in the circuit becomes 15,000 ohm (10,000 + 5,000), so the current changes to 10/15,000 or 0.00067 A, (0.67 mA). The current value is dropped 30%. Obviously, that is a significant difference.

On the other hand, if the meter internal resistance is only 50 ohm the total circuit resistance will be 10,050 ohm and the current will be equal to 10/10,050, or 0.000995 A (0.995 mA). This is within 0.05% of the correct nominal value. Clearly, for an accurate current reading, the ammeter internal resistance must be as low as possible.

Because an ammeter must be used in series with the circuit it’s testing, one of the connections in the circuit has to be physically disconnected, so the ammeter can be inserted. Often this requires desoldering.

Quite often it’s necessary to measure a current that is larger than the available meter with handle. This problem can be taken care of with a shunt resistance, that is, a resistor in parallel with the meter movement. See FIG. 4.

- --4 Shunt resistance across an ammeter.

By carefully selecting the proper ratio between the shunt resistance and the internal resistance of the meter itself, you can measure virtually any amount of cur rent flow. Most ammeters are actually milli-ammeters or micro-ammeters with an appropriate shunt resistance, because the tiny coil in a meter movement usually can’t carry a full ampere without damage.

The shunt resistance is generally quite small. As an example, suppose you have a meter with an internal resistance of 50 ohm and which has a full-scale reading 011 mA (or 0.001 A). This means that a current of 1 mA will cause maximum deflection of the armature and pointer. A greater current might damage the meter.

Now, suppose you need to use this meter to read currents up to 100 mA (0.1 A). The meter itself can handle only 1% of the desired full-scale reading, so, obviously, the shunt resistance will have to carry the other 99%.

Using Ohm’s law you can find the full-scale voltage dropped through the meter. E = IR, or, in this example, 0.001 A x 50 ohm or 0.05 Vt. For a full-scale reading of 0.1 A, the shunt will have to carry a current of 0.099 A (0.001 + 0.099 = 0.1). Because the shunt resistance is in parallel with the meter, the voltage drop will be the same. Therefore, R = E/I = 0.05/0.099, or approximately 0.5 (1. Notice that this drops the apparent resistance of the meter to 1/50 + 1/0.5, or 0.495 Ohm-meter.

From the above example, note that the full-scale reading of an ammeter cannot be increased by too large a factor, or the shunt resistance will become impracticably small. In fact, this particular example would probably be rather impractical, because resistance values below 10 ohm are fairly rare.

Of course, when the range of a meter is changed, the calibration markings on the scale face will no longer be accurate. However, when the increased full-scale reading is an exact multiple of 10 (as in the example) the same calibration markings can be used, and the appropriate number of zeros can be added mentally. For in stance, if the meter in the example gave a reading of 0.5 mA on its old scale, it would mean that the actual current was 0.5 x 100, or 50 mA (0.05 A).

Of course, all shunt resistors should have the tightest tolerance possible. Resistors with a 1% tolerance are generally essential, but 5% resistors can be adequate in some uncritical applications. The tolerance should never be more than 5%.

Voltmeters

To measure the current flowing in a circuit, the meter is placed in series with the circuit. If, on the other hand, a meter is connected in parallel across a resistance within a circuit (such as in FIG. 5), the deflection of the pointer will be proportional to the voltage drop across the resistance.

- --5 A voltmeter in parallel with the circuit to be measured.

Actually, the meter is still measuring the current flowing through it, but because current is determined by voltage and resistance (I = E/R), making the resistance constant, will cause the current through the meter to vary in step with the voltage. The meter’s scale can easily be calibrated so that the reading is given directly in volts. Such a device is called a voltmeter.

In an ammeter, the resistance should always be kept as low as possible, because it’s in series with the circuit being tested, because a large resistance would produce a large voltage drop that would not be present if the meter was not in the circuit.

In a voltmeter, on the other hand, the resistance should be as large as possible. Generally, a rather large fixed resistor called a multiplier is connected in series with the meter, as shown in FIG. 6. This multiplier resistance serves two important functions—it protects the meter movement from excessive current, and it helps pre vent circuit loading.

- --6 Adding a multiplier resistance to a voltmeter.

Suppose you want to take measurements in the simple resistor/voltage source circuit shown in FIG. 7A Assume each of the two resistors has a value of 1000 ohm and the voltage source is 10 V. The total resistance in the circuit is 1000 + 1000 or 2000 ohm. The current in the circuit equals 10/2000, or 0.005 A (5 mA), which means the voltage drop across each resistor equals 1000 x 0.005, or 5 V.

Now, when you connect the meter, as in FIG. 7B, it places a resistance in parallel with the resistor. The equivalent circuit is shown in FIG. 7C.

If the meter has an internal resistance of 50 the parallel combination of the meter and R_L is found by the formula, 1/RT = 1/R_1 + 1/Rm = 1/1000 + 1/50 = 21/ 1000 = approximately 48 ohm.

Now, the total effective resistance in the circuit is just 48 + 1000, or 1048 ohm The current is changed to 10/1048, or about 0.01 A (10 mA). The voltage drop across the R1—Rm combination is 0.01 X 48, or a mere 0.48 V. This reading will be the reading shown on the meter. Quite obviously, this is extremely inaccurate.

- --7 A simple circuit. The basic circuit (A); the circuit with a voltmeter added (B); equivalent circuit of the circuit and the meter (C).

Now, suppose you include an extra series resistor (that is, a multiplier) inside the meter, so that the total resistance of the meter is 20,000 ohm. In this case, the parallel combination of R and RM can be calculated as 1/RT = 1/1000 + 1/20,000 = 21/20,000 or about 952 ohm. This is much closer to the original value of 1000 ohm.

If you increase the meter resistance further to 1,000,000 ( (1 megohm), the parallel combination value comes even closer to the nominal value of R alone—just slightly over 999.

For the most accurate readings a meter should be essentially non-existent as far as the circuit’s operation is concerned. For a voltmeter, increasing the resistance of the meter will increase the accuracy of the measurement.

However, if you’re comparing voltages between similar circuits (as in servicing, when you compare a defective unit to a working standard), the meters used in each circuit should have the same internal resistance, or identical voltages won’t pro duce the same readings. If the difference is large, you might not be able to compare the readings in any meaningful way.

Ohmmeters

The third type of common meter circuit measures dc resistance and is called an ohmmeter. Of course, the meter itself is actually responding to the current flowing through it, but by passing a known voltage through an unknown resistance, you can measure the current and use Ohm’s law to calculate the resistance. You don’t have to actually do any calculations when you use an ohmmeter—the scale is calibrated to read the resistance directly in ohm.

The basic ohmmeter circuit is shown in FIG. 8. The resistor labeled Rm is included to calibrate the meter. For example, if the battery is 3 V, and the total be tested resistance of the meter circuit is 3000 Ii and you short the test leads together (0 f external resistance), the current flowing through the meter will equal 3/ (3000 + 0), or 0.001 A (1 mA). If the meter has a full-scale reading of 1 mA, the pointer will move all the way over to the high end of the scale, which is marked 0 for an ohmmeter. Rm is used to aim the pointer directly at the zero mark when the leads are shorted. As the battery ages, the required resistance to do this will change.

- --8 A basic ohmmeter circuit.

On the other hand, if the test leads are disconnected (infinite external resistance) no current at all will flow through the meter, because there isn’t a complete circuit path available. Of course, the pointer will remain at its rest position (low end of the scale). On an ohmmeter, this position is labeled o, or infinity.

Now, if you connect a 3000 ohm resistor between the test leads, the total circuit resistance will equal 3000 (internal resistance) + 3000 (external resistance), or 6000 ohm. Therefore, the current flowing through the meter must equal 3/6000, or 0.0005 A (0.5 mA). The pointer will move the center of the scale.

You’ll notice that the lower the resistance between the test leads, the farther the pointer moves up the scale. This, of course, is the exact opposite of what happens with voltmeters and ammeters, where the pointer moves further for higher values.

There is another major difference in the way the scale of an ohmmeter must be calibrated. This difference is indicated in Table 12-1, which compares the current flow with the test resistance. Unfortunately, the current through the meter does not change in a direct linear fashion with the external resistance. This nonlinearity is present because the internal resistance is, of necessity, a constant value.

----Table 12-1. Why an ohmmeter scale is not linear.

E R_M Test resistance RT I(mA)

3 3000 0 3000 1.00

3 3000 500 3500 0.86

3 3000 1000 4000 0.75

3 3000 1500 4500 0.67

3 3000 2000 5000 0.60

3 3000 2500 5500 0.55

3 3000 3000 6000 0.50

3 3000 3500 6500 0.46

3 3000 4000 7000 0.43

3 3000 4500 7500 0.40

3 3000 5000 8000 0.38

3 3000 5500 8500 0.35

3 3000 6000 9000 0.33

3 3000 6500 9500 0.32

3 3000 7000 10000 0.30

3 3000 7500 10500 0.29

3 3000 8000 11000 0.27

3 000 8500 11500 0.26

3 3000 9000 12000 0.25

3 3000 9500 12500 0.24

3 3000 10000 13000 0.23

3 3000 10500 13500 0.22

3 3000 11000 14000 0.21

3 3000 oo oo 0.00

The internal resistance can be altered to change the total range of the meter (if the internal resistance is 10,000 Z a mid-scale reading would indicate an external resistance of 10,000 11), but within a given range, the resistance is fixed as far as the actual testing is concerned.

AC meters

An ohmmeter can only be used to test dc resistance—not reactance or impedance. It might seem that an ac ohmmeter could be built with an ac power source, using a circuit something like the one shown in FIG. 9. However, any measurement made with such a device would be meaningful only at the specific frequency of the source voltage used in the test. The voltage source could be made so that the frequency is variable, but such a circuit would add greatly to instrument cost and complexity. Testing would be a long, drawn-out process. Besides, such a device would give no indication of phase relationships.

- --9 A theoretical ac ohmmeter. Impendence to be tested

Unfortunately, there is no easy way to directly measure impedance. Usually it must be calculated from voltage and current measurements. Fortunately, meters can be constructed to read ac voltages and currents, with the help of a device called a diode, which only lets current pass through it if the polarity is correct. If the polarity is reversed, the diode blocks the current. FIG. 10 shows what happens to an ac signal as it passes through a diode. Only half of the actual ac waveform is applied to the meter itself. Because the two halves of an ac waveform are mirror images of each other, the total value can easily be derived. Diodes are discussed in detail in another section. Meters for ac generally measure rms values of sine waves. If the waveform is something other than a sine wave, the reading won’t be equal to the true rms value.

- --10 What happens to an ac signal when it passes through a diode.

VOMs

Probably the most commonly used piece of equipment in electronics work is the VOM. VOM stands for volt-ohm-milliammeter. Various resistors are switched in and out of series or parallel to set up the meter for each type of measurement. An internal battery is included for resistance measurements. Some VOMs don’t have the capability to measure current directly, and many are designed for dc use only.

Usually any of a number of different value resistances can be switched into the circuit, so measurements can be made within different ranges. Whichever range is easiest to read for a specific value can be easily selected, 10 mV would probably be very difficult to detect on a meter scale that went up to 100 V.

The scale face of a VOM has a number of sets of calibration markings, so each of the metering functions can be read directly. Usually the different ranges are multiples of 10 of the basic range, so converting the reading to the appropriate range is simply a matter of mentally adding the correct number of zeros. For example, a reading of 1.5 on an x 100 range would indicate a value of 150.

Closely related to the VOM is the VTVM, or vacuum-tube voltmeter. This device is operated by an ac power supply and has an extremely high input impedance, and therefore it has a very high degree of accuracy. The disadvantages of the VTVM are its greater cost and complexity, and the fact that it must be plugged into an ac wall socket, which limits its portability.

A VTVM’s sensitivity and high impedance can be simulated by a special type of VOM that is built around a device called afield-effect transistor, or FET This component is dealt with in another section.

In the last decade or so, meter-type VOMs have been largely (but not completely) replaced by digital multimeters, or DMMs. These devices are not built around a mechanical meter. Instead, the value of the measured signal is displayed directly in numerical form using LEDs (light-emitting diodes) or LCDs (liquid-crystal displays). These display devices will be covered in another section of this book.

For most applications, DMMs and VOMs or VTVMs are pretty much interchangeable. DMMs usually have very high input impedances, so they are quite accurate. In addition, the numerical display is easier to read than a meter pointer. There is no need to worry about any error from looking at the meter face at an angle. So why haven’t DMMs replaced mechanical meters altogether? For some applications they really don’t work very well. This problem is especially noticeable when you are measuring values that change over time ( For example, the charge on a capacitor). Although the movement of a meter pointer is easy to follow either up or down (or even back and forth), a DMM will just display an unreadable and meaningless blur of rapidly changing numbers. A well-stocked modern electronics workbench will have both a VOM (or VTVM) and a DMM.

QUIZ

1. Which of the following cannot be easily measured with a simple meter circuit?

A Resistance

B Current

C Impedance

D Voltage

E None of the above

2. What is the most common type of meter movement?

A D’Arsonval

B Fixed coil

C Digital

D Farad

E None of the above

3. What type of meter is used to measure current?

A Ohmmeter

B Wattmeter

C Moving-coil meter

D Ammeter

E None of the above

4. How much internal resistance should an ammeter have?

A As much as possible

B A variable amount determined by Ohm’s law

C As little as possible

D It doesn’t matter

5. To increase the capacity of an ammeter, what should be added to the circuit?

A A series resistance

B A shunt resistance in parallel with the meter

C A shunt capacitance in parallel with the meter

D A series inductance

E None of the above

6. How is a voltmeter used?

A It’s placed in parallel across the component being measured

B It’s placed in series with the component being measured

C It’s placed within the magnetic field of the component being measured

D None of the above

7. Which type of meter requires its own power source?

A A voltmeter

B An ohmmeter

C A wattmeter

D An ammeter

E None of the above

8. What is a VOM?

A A combination voltmeter and ohmmeter

B A voltage only meter

C A combination ohmmeter, milliammeter, and voltmeter

D A measurement of the movement of a meter’s pointer

E None of the above

9. For the greatest accuracy, what should the input impedance of a VOM be?

A 50,000 Il/V

B 1000 ohm/V

C As large as possible

D As small as possible

E 1,000,000 ohm/V

10. What do ac voltmeters measure?

A The peak voltage of a sine wave

B The rms voltage of any waveform

C The average voltage of a sine wave

D The rms voltage of a sine wave

E None of the above

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